Insects exhibit a range of mouthparts, adapted to particular modes of feeding. The earliest insects had chewing mouthparts. Specialization has mostly been for piercing and sucking, although a range of specializations exist, as these modes of feeding have evolved a number of times (for example, mosquitoes (which are flies) and aphids (which are true bugs) both pierce and suck, however female mosquitoes feed on animal blood whereas aphids feed on plant fluids). In this page, the individual mouthparts are introduced for chewing insects. Specializations are generally described thereafter.
Like most external features of arthropods, the mouthparts of hexapoda are highly derived. Insect mouthparts show a multitude of different functional mechanisms across the wide diversity of species considered insects. Significant homology is conserved, with structures being formed from the same basal cell lines, and having the same evolutionary origin. On the other hand, even analogous structures may not share true homology, and are only easily comparable due to convergent evolution.
Chewing insects have two mandibles, one on each side of the head. The mandibles are positioned between the labrum and maxillae. They are typically the largest mouthparts of chewing insects, being used to masticate (cut, tear, crush, chew) food items. They open outwards (to the sides of the head) and come together medially. In carnivorous chewing insects, the mandibles can be modified to be more knife-like, whereas in herbivorous chewing insects, they are more typically broad and flat on their opposing faces (e.g., caterpillars). In male stag beetles, the mandibles are modified to such an extent that they do not serve any feeding function, but are instead used to defend mating sites from other males. In ants, the mandibles also serve a defensive function (particularly in soldier castes). In bull ants, the mandibles are elongate and toothed, used as hunting (and defensive) appendages. In bees, which feed primarily by use of a proboscis, the primary use of the mandibles is to manipulate and shape wax, and many wasps have mandibles adapted to scraping and ingesting wood fibres.
Situated beneath the mandibles, paired maxillae manipulate food during mastication. Maxillae can have hairs and "teeth" along their inner margins. At the outer margin, the galea is a cupped or scoop-like structure, which sits over the outer edge of the labium. They also have palps, which are used to sense the characteristics of potential foods.
The labium is a quadrupedal structure, although it is formed from two fused secondary maxillae. It can be described as the floor of the mouth. With the maxillae, it assists manipulation of food during mastication or chewing or, in the unusual case of the dragonfly nymph, extends out to snatch prey back to the head where mandibles can eat it.[better source needed]
The hypopharynx is a somewhat globular structure, arising from the base of the labium. It assists swallowing.
This section deals only with sucking insects, not those that pierce prior to sucking. The typical example is the moths and butterflies, although as is always the case with insects, there are variations. Some moths have no mouthparts at all. All but a few adult Lepidoptera lack mandibles (the mandibulate moths have fully developed mandibles as adults), with the remaining mouthparts forming an elongated sucking tube, the proboscis.
One of the more defining characteristics of lepidopterans is their coiled proboscis. It is held coiled under the head when not in use. During feeding, however, it is extended to reach the nectar of flowers. The proboscis is a long tube that is formed by heavily modified maxillae, specifically the galea.
Piercing and sucking insects
A number of insect orders (or more precisely families within them) have mouthparts that pierce food items to enable sucking of internal fluids. Some are herbivorous, like aphids and leafhoppers, while others are insectivorous, like assassin bugs and mosquitoes (females only).
The defining feature of the order Hemiptera is the possession of mouthparts where the mandibles and maxillae are modified into a proboscis, sheathed within a modified labium, which is capable of piercing tissues and sucking out the liquids. For example, true bugs, such as shield bugs, feed on the fluids of plants. Predatory bugs such as assassin bugs have the same mouthparts, but they are used to pierce the cuticles of captured prey.
In female mosquitoes, all mouthparts are elongated. The labium encloses all other mouthparts like a sheath. The labrum forms the main feeding tube, through which blood is sucked. Paired mandibles and maxillae are present, together forming the stylet, which is used to pierce an animal's skin. During piercing, the labium remains outside the food item's skin, folding away from the stylet. Saliva containing anticoagulants, is injected into the food item and blood sucked out, each through different tubes.
The housefly is the typical sponging insect. The labium gives the description, being articulate and possessing at its end a sponge-like labellum. Paired mandibles and maxillae are present, but much reduced and non-functional. The labium forms a proboscis which is used to channel liquid food to the oesophagus. The housefly is able to eat solid food by secreting saliva and dabbing it over the food item. As the saliva dissolves the food, the solution is then drawn up into the mouth as a liquid.
The labellum's surface is covered by minute food channels, formed by the interlocking elongate hypopharynx and epipharynx, which form a tube leading to the oesophagus. The food channel draws liquid and liquified food to the oesophagus by capillary action.